Method of and apparatus for producing electrical signals representing graphic information



Feb. 23, 1965 DQF. O'BRIEN 3,17 7 umaon or mo APPARATUS FOR Paonucmc ELECTRICAL SIGNALS REPRESENTING GRAPHIC INFORMATION Filed D60. 12, 1963 WRITING x COIL x con. m a

seusms sEcnoNs OF COILS Lil Y con.

FIG. 2

ATTORNEY United States Patent 3,170,987 METHOD OF AND APPARATUS FOR PRGDUtIlNG ELECTRICAL SIGNALS REPRESENTING GRAPH- IC INFORMATION Donald F. OBrien, 55 Fairview Ave, Watertown, Mass. Filed Dec. 12, 1963, Ser. No. 331,339 Claims. (Cl. 178-18) This invention relates to methods of and apparatus for producing electrical signals representing graphic information and particularlyto methodsof and'transducers for producing voltages which are functions of the movement of a stylus.

This application is a continuation-in-part of my copending application Serial No. 146,719, filed Oct. 23, 1961, now abandoned, entitled Graphic Information Transducer.

Transducers of the general type with which the present invention is concerned aredesigned to introduce graphic information, in the form of a movement of astylus', into an electrical system, primarily for the purpose of transmitting the information to a-location remote from thetransducer where it can be reproduced. Graphic communication systems of this'type, also known as' telcautographic systems, generally comprise a transducer including a planar Writing surface and a stylus, an amplifier and a recording or reproducing device.

An object of the invention is to provide a novel and improved transducer for introducing graphic information into an electrical system, and a method utilizing said apparatus for producing electrical signals representing graphic information.

Heretofore, transducers employed in graphic communications systems have been eitherof 'two general types. In one type the stylus is connected through a mechanical linkage, such as a pantograph, to'sorne type of electrical I means such as a variable resistance or capacitance for varying'the parameters of the electrical system. The

3,lid,987 Patented Feb. 23, 1965 Ice l lar 'to the adjacent turns of the other coil. This ideal condition of perpendicularity may be difiicult to achieve in actual practice and where turns ofone coil are not perpendicular toadjacent turns of the other coil, a portion of the voltage generated in one coil by virtue of the motion of the stylus and its associated magnetic field will be coupled into the other coil by transformer action and result in the induction in the other coil of spurious volt ages, that is, voltages which-are not the result of motion of the stylus. 7

Still another object of the invention is to provide in a transducer ofthe'type described, a novel circuit for cancelling out spurious voltages generated in the coils as a result of transformer action between the coils. 1 Other objects of the invention will in part be obvious and will in part appear hereinafter.

mechanical coupling between the stylus and the electrical system not only represents a source of inaccuracy, error, and distortion of the information during transmittal thercof between the stylus and electrical system, but the linkage itself tends to introduce distortion into the motion of the stylus. The other type of prior art systems sufier from the disadvantage that the voltage produced is dependent upon an electrical contact between different elements such as the movable stylus and a conductive writing surface or turns of two coils, the resistance of which contact is, of course, subject to variation. 7 V r The invention'has as other objects the provision of a graphic information transducer the operation of which is independent of mechanical coupling between the stylus and other components of the transducer, or electrical coupling between conductors and is designed to introduce graphic information with substantially less error and distortion and with greater resolution than has been possible heretofore; to provide a transducer as described comprising a stylus having a substantially constant magnetic field associated therewith, a pair of coils having sensing portions lying in parallel overlying planes with the turns of the sensing portions of each coil perpendicular to the turns of the other and means supporting the stylus for movement in a plane parallel with the plane of the sensing portions; and to provide a transducer of the type described in which the magnetic stylus is constructed to minimize the eifect of variations in the angle of the stylus with respect to the plane of the sensing portions of the coils.

The invention accordingly comprises. the process involving the several steps and the relation and order of one or more of vsuchsteps'with respect to each of the others and the apparatus possessing the construction, combination 7 ofelements and'arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a further understanding of the nature and .objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:

FIGURE 1 is a schematic electricaldiagram illustrating the construction and operation of a transducer embodying the invention and the method thereof;

FIG. 2 is a diagram illustrating somewhat schematically, the arrangement of coils comprising the transducer; FIGS. 3 and 4 are front and side elevational views showing, in section, an arrangement of coils comprising the I transducer;

eliminating spurious voltages caused by transformer coupling between the coils of the transducer.

Another characteristic feature of prior art transducers is that they function as integrators and produce voltages commensurate with the magnitude of the plane-rectangular or Cartesian, coordinates of position of the stylus. This information, while being suitable for controlling the opera- The .transducer embodying the invention comprises a substantially plane, rectangular Writing surface covering sections of a pair of coils designated as an X coil and a Y coil. Each of the coils comprises a single conductor having a larger multiplicity of turns, sections of which are arranged in substantially parallel relation and lie in a plane. These sections of the coils lying in planes and termed the sensing sections, are arranged with the turns thereof in substantially perpendicular relation in the form of planerectangular X and Y coordinates, i.e., with the turns of the sensing section of the X coil extending parallel with the Y coordinate axis, and the turns of the sensing section of theY coil extending parallel with X coordinate axis. The planar sensing sections of the coils are arranged in overlying, contiguous, parallel relation closely adjacent and parallel with the above-mentioned writing surface; and in a preferred form of the invention may be imbedded in the material comprising the writing surface.

In an alternative embodiment, each of the coils is provided with two sensing sections each lying substantially in til a plane and cooperating to effectan increase in the'output potential of each coil. The other component of the transanvase'r ducer is a stylus comprising a permanent magnet which is coils, a current is generated within that coil having a voltage commensurate with the speedof movement of the stylus in a direction perpendicular to the turns of the sensing section of the coil. Thus the voltage Yx, developed in the X coil can be expressed as a constant K times the velocity dX/zz'yin the Y direction. Similarly, the voltage Vy, developed in the Y coil, is expressed as (K) '(dx/dt) In this manner any movement of the stylus is represented by two voltages, the magnitudes of which are derivatives of the motion of the stylus in the X and Y directions. The graphic information is introduced into the'transducer as a movement of a magnetic stylus and is converted by the transducer into two-electrical signals representing derivatives of that movement. This method of producing electrical signals representing graphic infor mation is particularly suited to-the introduction of graphic information into acomputor; Information input is more efficient and. the memory requirements are reduced because constant monitoring and sampling of the signal is not required, since only changesin the signal require attention. These and other advantages of representing graphic information in a derivative form should 1.15s

readily appreciated by one familiar with computors, for example, -of the stored program digital type. The system of the invention has the additional advantages of: substantially infinite resolution since the signals are not dependent upon the size or location of a conductor or. location and quality of an electrical contact; and substantially unlimited size because neither the electrical resistance of lengthy conductors, the strength and extent of a magnetic field, nor. the dynamic range of devices (electronic) such'as amplifiers, integrators, and'the like,

with whichthe transduceris coupled, is limiting factor. In orderto reproduce the graphic information, to,

cause a recording instrument to duplicate the motion of the magnetic stylus, the derivativevoltages are integrated to produce voltages which are commensurate with the magnitudes of the rectangular coordinatesof position of the stylus; and these voltages maybe employed to control the operation of a conventional X-Y recorder or other indicating or recording device. Integrating circuits, X-Y

recorders and indicating devices such as oscilloscopes useful for this purpose are well known in the art and easily adapted to use in conjunction with the transducer of the invention. 1 a

In constructing and assembling the transducer, care is required to insure that adjacentturns of the two coils are perpendicular to one another in order 'to' prevent coupling of the coils by transformer action in which the current flow in one coil, resulting from motion of the stylus, induces a spurious voltage in the other coil. Of course, parallelism in turns of the planar sensing sections of each'coil and'perpendicularity of the turnsof the two coils are also important in order to insure uniformity, accuracy and freedom from distortion of the information. FIG. 2 illustrates one arrangement of the coils in which each of the coils is Wound in a form of a substantially planar, ,open rectangle, and the two coils arearranged with perpendicular, sections of-the'coiloverlapping one another at 'two locations. 'Theaplane writing surface, which is preferably rectangular-(and shown as a square),

is located at one of the two locations where sections of the coils overlap. Other arrangements are possible and include, for example, winding the coils around a support member in the form of a prism having five faces including a rectangular face which underlies the writing surface and supports the sensing sections of the coils, two other rectangular faces on which the return sections .of both coils are supported in perpendicular relation, and two opposed triangular faces on which the return sections of one of the coils are supported.

Another arrangement comprises winding the coils arounda support member in the form of a thin parallelepipedon. A preferred form of this arrangement is illustrated in FIGS. 3 aud t-and comprises means for magnetically shielding the return sections of the two coils from one anothenfrom the planar sensing sections of the coils and from external magnetic effects. This is accomplished by the provision of shielding panels formed of a high permeability, magnetic barrier alloy such as mumetal or alloys sold by Perfection Mica Co. under the trade names Netic and Co-netic. The panels include a first spacer panel 19 lying between the planar sensing sections of the coils and the return section of the underlying or inner coil, designated for the purposes of example as the Y coil; a second spacer. panel 12 disposed between the return section of the one (Y). coil and the return section of the other (X) coil; a bottom panel 14 disposed outside ofthe return section oflthe.

X coil; and four side panels 15 disposed around'the sides of the two coils. 'The spacer panels function to prevent any unwanted coupling by transformer'action involving the return portions of the coils, while the bottom and i side panels prevent the development of spurious voltages dueto external magneticfields. c As previously noted, the stylus may be held in the hand andmoved, in the manner of an ordinary pen or pencil, in

' contact with the Writingsurface which may, if desired,

be formed of a materialupon which the stylus will form a visible image. SinceIthere is, of course, no way .of precisely controlling the angle at which the. stylus is held with respect to the writing surface and the plane of the sensing sections of thecoils without inhibiting the natural writing motion of the operator, the end of the stylus which is held in contact with the writing surface is con-. structed sothat the associated magnetic field assumes a form which eliminates or at least minimizes the effect of changes in thestylus angle on the voltage induced in v the coils. This is accomplished, as shown in FIG. 5, by

employing a cylindrical bar magnet having a tapered end in the form of a cone of revolution of an are having a radius, designated R, which is 2 to 4 times the diameter of the stylus. The preferred radius of this are has been determinedempirically as being of the order of three times the diameter of the magnetic stylus.

Itmay be very difficult, as a practical. matter, to wind' the two coils in sucha manner that all the turns of each coil are perpendicular to all the turns of the other coil, and for this reason some transformer coupling may exist between the coils resulting in the development of spuriousvoltages'which cause distortion. A circuit for eliminating this distortion caused by transformer action between the coils is illustrated in PEG. 6. The

circuit shown is designed to cancel out the voltages induced in one coil as aresult of the flow of current generated in the other coil by developing a voltage at each coil which is equal to the spurious voltage and is of opposite sign; i.e., out of phase. In the circuit shown, the X and Y coils, having the polarity shown, are connected to means such as amplifiers (not shown), and

toground. Each of the coils is connected through a pair of resistors R and R to one winding ofa transformer.-

Each of resistors'R is connected serially with a sensing coil and one end of a transformer windingg while each of resistors R is connected serially wtih azsensing coil, resistor R and theopposite end of the transformer coil anions? which is grounded. Resistors R and R are provided for adjusting the parameters of the circuit so that the transformer develops at each of the sensingcoils a voltage equal to and oppositein sign, to the voltage induced in each coil as a result'of coupling of the coils by transformer action. In a transducer in which the operating'frequencies are relatively. low, for example, 30 cycles per second, the values of the resistors may be determined in accordance with the equation in which M is the percentage of the voltagein one coil induced by transformer action in the other coil, R andv and a transformer fora turns ratio of one, so that the.

selection of the proper values for the resistances will resultin bilateral cancellation of spurious voltages. For non-square (rectangular) writing surfaces, bilateral cancellation ofspurious voltages can be achieved by proper selection of either the diameter of the conductors comprising the sensing coils or the turns ratio of the transformer.

The operation of the transducer, like its construction, is quite simple, requiring only'that the operator hold the stylus as if it were an ordinary 'pen or pencil and go through the motions of writing with the stylus in'contact with the writing surface overlying the sensing sections of the coils. The stylus is free from the incurnbrance of any of the mechanical linkagescharacteristic of many prior art systems, so that the movements of the stylus are unhampered and natural; and the coupling between the stylus and coils is magnetic rather than ohmic, and accordingly, is substantially more constant particularly when the stylus is shaped as shown and described in order to minimize or eliminate any effects resulting from changes in angle subtended by the axis of the stylus and the plane writing surface. All of the foregoing features cooperate in achievement of a transducer which is especially free of factors tending to cause distortion, inaccuracies and poor resolution and is capable of more faithful reproduction of the graphic information particularly handwriting being introduced.

Since certain changes may be made in the above apparatus and method without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A graphic information transducer for producing electrical signals characterizing the motion of a member in a plane, said transducer comprising, in combination:

a pair of independent coils each having a sensing section with substantially parallel turns lying in a plane, said coils being arranged with said sensing sections separated from one another and held stationary in overlying face-to-face relation and with said turns of each of said sensing sections substantially perpendicular to said turns of the other of said sensing sections;

an input member having an associated magnetic field which is substantially unvarying; and

means providing a planar surface substantially parallel with said planes of said sensing sections for supporting said input member for movement in the plane of said surface relative to said sensing sections with said magnetic field intersecting said sensing sections; said input member being freely movable in said plane of said surface for inducing a voltage in each of said coils commensurate with the speed of movement of said input member relative to said each coil in a direction perpendicular to said turns of said sensing section thereof and constituting one signals. a v p 2. The transducer ,of claim 1 in which said input member comprises an elongated cylindrical magnet having a tapered end section ending in a point and adapted to be held and moved manually with said point in contact with said surface. g

3. The transducer of claim 2 in which said tapered end section is a cone of revolution of an are having a radius which is 2 to 4 times the diameterof said magnet.-

4. The transducer of claim 'l in which each' of said coils includes a return section underlying said sensing section, and magnetic barriers are providedbetween said returnsections and between said sensing sections and 831d return sections.

5. The transducer of claiml in which each of said coils is coupled with one Winding of a transformer which produces a voltage at said each coil equal toand opposite in sign to the volta ge induced in said each coi-l by transformer coupling between said each coil and the other of said coils. 6. A transducer for introducing graphic information into an electrical system, said transducer comprising, in combination: Y

a pair of coils each having'a sensing section,,,the turns of which are substantially parallel and lie substantially in a plane; I v saidcoils being arranged with said sensing sections in overlying face-to-face relation and with said turns of each of said sensing sectionssubstantially perpendicuiar to said turnsjof the other .of said sensing sections; and means for providing a magneticfield;

said means being freely movable in, two dimensions in it a plane substantially parallel with the planes of said sensing sections with the latter located Within said magnetic field for inducing ,a voltage in each of said coils commensurate with the speed of movement of said magnetic field relative to and in a directionperpendicular to the turns of said sensing section of said .each coil; Y 7 each of said coils being grounded at one end and being coupled at its other end to a system, into which said graphic information is being introduced, through a pair of resistances and a winding of a transformer which produces a voltage at said each coil equal to and opposite in sign to the voltage induced in said I each coil by transformer coupling between said each one end of said transformer winding and the otherv of said resistances being'coupled serially with said each coil, said one resistance and the opposite end of said transformer winding which is grounded. 7. A sensing device for graphic information transducer comprising, in combination:

means providing a substantially planar writing surface; a first coil having a substantially rectangular sensing section including a multiplicity of turns arranged in parallel relation in a plane and positioned underlying, closely adjacent and parallel with said writing surface; and a second coil having a substantially rectangular sensing section including a multiplicity of turns arranged in parallel relation in a plane and positioned underlying, closely adacent and parallel with the plane of said first coil and said writing surface; 7 said sensing sections of said coils being substantially tions; 7 each of said coils being grounded at one end and con nected at its other end to an electrical system, into of said electrical tion:

which graphic information is to be introduced, 1

through a pair of resistances to a winding ofa transequal to and opposite in sign to the voltage induced in said each-coil by transformer coupling betweensaid each coil and the other of said coils, one of said resistances being coupledseriallylwith said each coil and one of said transformer windings and the a other of said resistances being coupled seriallywith said each coil, saidlone resistance and the opposite endofsaid-transformer'winding. t

8. A graphic information transducer for producingtwo voltages having magnitudes representing V derivativesLof the motion of a member in'rnutu-ally perpendicular directions in a plane, said transducer comprising, in combinaa first coil having a sensing section including a multiplicity of turns arranged in side-by-side', parallel relation substantially in afixed plane} a second coil having a sensing 'section including a multiplicity of turns arranged-in side-by-side, parallel relation substantially in a fixed plane closely adjacent and'in face-to-face. relation with said 'plane of said'sensing section of said first coil; T

said sensing sections being arranged in fixed positions.

with said turns ofieach of said sections perpendicular to said turns of the other of said sections; an input member having an associated magnetic field which is substantially unvarying; and a means provid n a planar surface substantially parallel jwithsaid planes of said sensing sections forsupporting said input member for movement in the plane of said surface relative to said sensing sections with said magnetic field intersecting said sensing sections;

said input member beingfreely movable in said plane 0 v i c: a a 9. In apparatus for reproducing the motion of a member in a plane, in combination:

a pair of independent coils each having a sensing section with substantially parallel turns lying in a plane, said coils being arranged with said sensing sect-ions separated from one another and held stationary in overlying face-to-face relation and with said turns of each of said sensing sections substantially perpen dicular to said turns of the other of said sensing sections;

an input member having an associated magnetic field which; is substantially unvarying;

means for providing a planar surface substantially parallel with said planes 'of said sensing sect-ions for supporting said input member formovement in the plane of said surface relative to said sensing sections;

said input member relative to said each coil in a direction perpendicular to said turns of said sensing;

section'thereof; and 7 means for integrating saidvoltages induced in said coils to produce two voltages representing the rectangular coordinates of position of said input member on-said surface; I 10. The apparatus of claim 9 in which each of said coils is coupledwith the last-mentioned means throughone winding of a transformer which produces a volt age at said each coil equal'tojand opposite in sign to the voltage inducedin said each coil by transformer coupling between said each coil and the other of said coils.

References Cited in the file of this patent UNITED STATES'PATENTS Y 2,586,160 Handel Feb. 19, 1952 2,907,824 Peek Oct. '6, 1959 

1. A GRAPHIC INFORMATION TRANSDUCER FOR PRODUCING ELECTRICAL SIGNALS CHARACTERIZING THE MOTION OF A MEMBER IN A PLANE, SAID TRANSDUCER COMPRISING, IN COMBINATION: A PAIR OF INDEPENDENT COILS EACH HAVING A SENSING SECTION WITH SUBSTANTIALLY PARALLEL TURNS LYING IN A PLANE, SAID COILS BEING ARRANGED WITH SAID SENSING SECTIONS SEPARATED FROM ONE ANOTHER AND HELD STATIONARY IN OVERLYING FACE-TO FACE RELATION AND WITH SAID TURNS OF EACH OF SAID SENSING SECTIONS SUBSTANTIALLY PERPENDICULAR TO SAID TURNS OF THE OTHER OF SAID SENSING SECTIONS; AN INPUT MEMBER HAVING AN ASSOCIATED MAGNETIC FIELD WHICH IS SUBSTANTIALLY UNVARYING; AND MEANS PROVIDING A PLANAR SURFACE SUBSTANTIALLY PARALLEL WITH SAID PLANES OF SAID SENSING SECTIONS FOR SUPPORTING SAID INPUT MEMBER FOR MOVEMENT IN THE PLANE OF SAID SURFACE RELATIVE TO SAID SENSING SECTIONS WITH SAID MAGNETIC FIELD INTERSECTING SAID SENSING SECTIONS; SAID INPUT MEMBER BEING FREELY MOVABLE IN SAID PLANE OF SAID SURFACE FOR INDUCING A VOLTAGE IN EACH OF SAID COILS COMMENSURATE WITH THE SPEED OF MOVEMENT OF SAID INPUT MEMBER RELATIVE TO SAID EACH COIL IN A DIRECTION PERPENDICULAR TO SAID TURNS OF SAID SENSING SECTION THEREOF AND CONSTITUTING ONE OF SAID ELECTRICAL SIGNALS. 